A typical semiconductor device usually includes an active region which includes a rectifying junction such as a PN junction or a schottky junction.
The breakdown down voltage of a rectifying junction is usually less than its theoretical limit because certain locations on the rectifying junction have a tendency to develop higher electric fields under reverse bias conditions. The rectifying junction at the terminal edge of the active region of a device, for example, experiences higher electric fields.
To reduce the intensity of the electric fields near the edge of a rectifying junction (e.g. PN junction), high voltage semiconductor devices may include an edge termination structure. An edge termination structure provides a transition region in which the high electric fields around the active area change gradually to the lower potential at the edge of the device. A termination structure lowers the field intensity around the termination region of a rectifying junction by spreading the electric field lines across the termination region.
Field plates with single or multiple guard ring structures are common termination techniques for low to mid voltage (15–200V) semiconductor devices such as trench Schottky diodes and MOSgated devices. In the trench Schottky diode such as the one disclosed in copending application Ser. No. 10/193,783, filed Jul. 11, 2002 (IR-1663), a recessed termination structure using a field plate is formed, as schematically shown in FIG. 1. In FIG. 1, a termination recess 10 is formed in epitaxially deposited substrate 11 by a trench etch. Field oxide 12 can be grown during the gate oxidation process, or deposited by a TEOS or LTO layer in a separate process step. After termination oxide 12 is grown, polysilicon is deposited to fill all trenches. The polysilicon is then doped and etched back so that spacer 13 is left on the sidewalls of termination region. Schottky layers 20 and contact metal layers 21 are next deposited and patterned to form a field plate overlapping termination oxide 12. The trench depth scales with the breakdown voltage: mid voltage (100V) trench devices usually require a deep trench (3˜5 μm), hence termination recess 10 which was produced at the same step as the trench etch for active area trenches 22 becomes very deep (˜6 μm for a 5 μm trench, due to the loading effect of the dry etch process). When photo resist (PR) 30 is coated in the metal mask process, it tends to get thinner at the poly shoulder (above the left hand spacer), and, in the metal etch process, photoresist 30 at the shoulder will eventually break. The etching solution will then penetrate the opening and undercut aluminum 21 and schottky barrier 20 metals in the active mesa region. This will introduce high yield loss during the manufacturing process.
U.S. Pat. No. 5,382,825 discusses a variety of termination structures and their respective drawbacks. To improve on the prior art termination structures discussed therein, U.S. Pat. No. 5,382,825 discloses a termination structure which includes a single spiral ribbon of resistive material disposed around the active region of a semiconductor device to gradually relieve the electric fields near the edge of the rectifying junction of the active region of a semiconductor device. FIG. 2 illustrates an example of a spiral termination structure. Endo in K. Endo et al., “A 500V 1A 1-Chip Inverter with a new electric field reduction structure”. Proc. ISPSD-94, pp. 379–383 also demonstrates the use of polysilicon as a two dimensional resistive layer on the top of an oxide layer.
The spiral ribbon disclosed by U.S. Pat. No. 5,382,825 is formed on a surface of the semiconductor device. As a result, if the spiral ribbon is widened to vary the resistance thereof the lateral expanse of the spiral is increased, and thus more die area is required resulting in a larger and more expensive die.
The concept of a spiral termination is described by Macary et al. and Krizaj et al. in V. Macary et al., “Comparison between biased and floating guard rings as the junction termination technique”, Proc. ISPSD-92, Tokyo, Japan, pp. 230–233 and D. Krizaj et al., “Diffused Spiral Junction termination structure: Modeling and Realization”, Proc. ISPSD-96, pp. 247–250. The leakage in the junction termination ring “partitions” the potential along the spiral ring and decreases the high electric field. However, an increase of the leakage current due to the hole injection appears before breakdown.